Bethesda mod archives

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The Gamebyro engine can load files from specialised archives into the game on startup. This allows DLCs and mods to package all their files together without worrying about them being accidentally overwritten and further simplifies the modding process by minimising the number of files. These archives can be either BSA (Morrowind, Oblivion, Fallout 3, Fallout New Vegas and Skyrim) or BA2 (Fallout 4 and Fallout 76) format files. Extracting or modifying archives should be considered an advanced modding technique and in most cases is unnecessary to allow the game to function correctly.


Mod archive compatibility

The BSA and BA2 archives are not automatically compatible with all games. Even among the variations of the same file type, they can cause unwanted side effects such as preventing the game from starting up. The table below lists the types of BSA and BA2 and how they behave with each Gamebyro title. The version of the archive can be determined by inspecting the file header. 

Game BSA v100 BSA v103 BSA v104 BSA v105 BA2 v1
Morrowind Compatible Incompatible Incompatible Incompatible Ignored
Oblivion Incompatible Compatible Ignored Ignored Ignored
Fallout 3 Incompatible Untested Compatible Untested Ignored
Fallout New Vegas Incompatible Untested Compatible Untested Ignored
Skyrim (2011) Incompatible Limited Support Compatible Incompatible Ignored
Skyrim Special Edition Incompatible Incompatible Incompatible Compatible Ignored
Fallout 4 Ignored Ignored Ignored Ignored Compatible
Skyrim VR Incompatible Untested Incompatible Compatible Ignored
Fallout 4 VR Ignored Ignored Ignored Ignored Compatible
Fallout 76 Ignored Ignored Ignored Ignored Compatible

Below is a more detailed explanation of each status. 

  • Untested - This combination has not been tested, feel free to update this article if you are able to test. 
  • Ignored - The game will not attempt to load this file. 
  • Compatible - This is the intended archive format for this game and is fully compatible. 
  • Limited Support - This archive type is not intended to be used with the game but does appear to work. The exact impact on loading times, stability or general performance is unknown. It is not recommended to continue to use this combination.
  • Incompatible - This combination will cause a critical error, usually crashing the game. 


Mod manager archive checking

Some mod managers include a feature that will automatically check archive versions and identify any mismatches. At the time of writing, only Vortex and Wryebash include this feature. 


Extracting and packaging archives

There are several community tools available to unpack and create new BSA and BA2 archives. At present no single tool can process all types of archive. Below is a list of tools that can be used in each instance. For community-created tools, you should refer to any documentation offered by the developer if you are unclear on how to use it. 

Archive Format Compatible Tools
BSA v100 (Morrowind)
BSA v103 (Oblivion)
BSA v104 (Fallout 3, New Vegas, Skyrim 2011)
BSA v105 (Skyrim Special Edition)
BA2 v1 (Fallout 4, Fallout 76)

Frequently Asked Questions (FAQ)

I'm getting a warning about incompatible mod archives, how can I resolve it?

A common mistake - particularly between Skyrim 2011 and Skyirm Special Edition - is to try and load a mod from another game or game version, ensure the mod you're trying to use is designed for the game you are trying to play.

If you are sure you have the correct mod, it's possible the mod author used the wrong tools when packing their archives (or the correct tools did not exist when the mod was made). In this scenario, the best thing to do is to extract the contents of the BSA/BA2 and delete it. You can see a list of tools to do this under #Extracting and packaging archives.

Note: Extracting a BSA v104 when playing with Skyrim Special Edition will prevent the game from crashing to desktop (CTD) but if the BSA contains incompatible texture or mesh assets this will still cause crashes when those resources are loaded. You should use Cathedral Assets Optimizer (or another compatible tool) to ensure all assets are game-ready.